Systems for inspecting containers such as those transported by truck or by boat use a high-energy photon radiation source.
FIG. 1 shows a perspective view of an exemplary embodiment of a container inspection device 10, of the prior art, towed by a haulage vehicle 12.
The inspection device of FIG. 1 essentially comprises a radiofrequency accelerator of electrons 20 hitting a target 22 which in its turn provides a radiation of high-energy photons 26 vertically sweeping a side of the container 10. The accelerator is excited by a microwave source 28 at a frequency f0.
A detector 30 placed on the other side of the container provided an image of a vertical slice of the content of the container. The displacement of the container 10 by the haulage vehicle 12 in a direction 32 makes it possible to obtain a complete image of the content over the whole length of the container. It is also possible for the detector and the container hauled by the truck to move in relative motion with respect to one another.
Other systems comprise two perpendicular sources of irradiation in one and the same inspection plane and two associated detectors so as to provide a (pseudo) three-dimensional image of the content of the container.
In this type of container inspection system, the radiofrequency accelerator is a linear accelerator or LINAC, for LINear ACcelerator, the trajectory of the electrons is always rectilinear, the electric field for accelerating the electrons is of high frequency.
The high-frequency sources used are almost always klystrons or magnetrons. The electrons are accelerated in the LINAC by appropriately synchronized successive high-frequency pulses. By passing through a series of cavities permeated by an alternating electric field, the beam may attain an energy of a few MeV.
Current systems for inspecting containers make it possible to achieve in the form of a series of energy pulses, either irradiations of photons with constant energy, or irradiations with changes of energy in “packets”, that is to say changes of energy over long durations with respect to an energy pulse.
The changes of energy on the linear accelerators of the prior art are based either on inter-section phase shifts, or on mechanical shunts making it possible to short-circuit the accelerator cavities at the end of the section. For a moderate span of energy variation, control of the beam current (or “beam loading”) or a moderate radiofrequency (RF) power reduction make it possible to change the energy of the electrons at the output of the LINAC but in a restricted span, typically a factor of two between the minimum energy and the maximum energy.
FIGS. 2a and 2b represent the energy of the electrons according to two techniques of pulse-based acceleration of the prior art using a radiofrequency accelerator of frequency f0.
FIG. 2a shows the energy of the electrons in the form of a series of pulses of width L and of energy E that is constant from one pulse to another during a certain time.
FIG. 2b shows the energy of the electrons in the form of successive packets P1, P2 of pulses of like width L. The energy of the pulses of each packet is the same silk E1 for the pulses of the packet P1 and E2 for the pulses of the packet P2.
In a known manner, the energy of the photos which is radiated by the target, expressed in MV, is directly related to the energy of the electrons, expressed in MeV, at the output of the acceleration radiofrequency device impacting said target.
In the prior art system a certain latency time Tr is required in order to go from the pulses of energy E1 to the pulses of energy E2, thereby representing a drawback for the inspection device. This latency time Tr is due, in prior art LINACS with switching, to the time required for mechanically switching the shunts so as to short-circuit certain elements of one of the cavities of the LINAC so as to vary the electric field in the cavities.
In prior art LINACS with two cascaded sections the latency time Tr is due to the time required for the change of phase in the output section by motors controlled by an energy changing device.
In current container inspection systems, it is sought to obtain an ever greater span of variation of the energy radiated so as to increase the precision of the identification of the content of a container.
The current demand leads to the investigation of inspection systems making it possible to achieve irradiations where the energy is changed from one pulse to another.